Add mechanism to force ECT computation value in NQE

Add an API to Network Quality Estimator that can be called
to force the computed Effective Connection Type value.
This will be used in a subsequent CL.

Change-Id: I96d3dea84af0c87ba841cf1115269ce7709a6e0c
Bug: 819244
Reviewed-on: https://chromium-review.googlesource.com/1107489
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568922}
5 files changed